It has long been known the technique of coating by flame spraying thermoplastic powders applied by melting. Such technique is used for example for the production of anti-corrosive coatings on manufactured articles of different nature.
According to a known method, the thermoplastic powders are sprayed on the manufactured article to be coated by means of a spray gun fed with compressed air and with a suitable liquefied petroleum gas. The gas flame produced by the spray gun transfers the melted particles of the powders on the article to be coated.
The method of coating by flame spraying is of rapid and economic use and is adapted for coating different materials. The apparatuses currently used to obtain such coating, however, have certain drawbacks which limit their performance and thus make the use of the aforementioned method less effective.
In particular, it is often complained the fact that the spray gun is subject to overheating during use. This may, inter alia, adversely affect the physical characteristics of the powders to be applied.
To solve this problem, patent application ITBO2009A000292 in the name of the applicant discloses a method for flame spraying thermoplastic powders which provides in particular to heat, firstly, the surface to be coated, and then mix the thermoplastic powders with an inert carrier gas, and project it through compressed air and/or nitrogen against the heated surface. The method also provides to feed a flow of liquefied petroleum gas for the production of a flame so as to maintain the surface to be coated at the desired temperature. Such patent application also discloses an apparatus adapted to carry out the above mentioned method, which provides in particular a spray gun provided with a mixing device, inside which the thermoplastic powders, the compressed air and the liquefied petroleum gas are mixed. The mixing device comprises three outlet chambers separated for each of the above mentioned components.
However, this solution still presents some drawbacks. In particular, it does not allow a perfectly homogeneous spraying of the thermoplastic powders on the surface of the article to be coated.
In fact, such solution provides a radially symmetrical flow, flowing out of the mixing device, in particular having a conical-cylindrical shape, not always uniform. The result is the formation on the processed article of accumulations of powders, concentrated on strips, under which pockets of air can also be trapped. These air pockets, besides giving a visual unattractive appearance to the coated article, also involve the risk of creating a non-continuity of the coating, which could lead to poor impact resistance and thus to a considerable fragility of the coating.